Leaves That Lunch

The most famous carnivore of the plant kingdom, the Venus flytrap, lures insects to its leafy green lips with a sweet-smelling scent, then snaps its mouth-like leaves shut in just 0.1 second to ensnare its prey. Scientists have long sought an explanation for how a plant, which has no muscles, can move fast enough to catch a fly. Lakshminarayanan Mahadevan, Gordon McKay professor of applied mathematics and mechanics, leading a cross-disciplinary team of mathematicians, engineers, and biologists, recently found the answer, which appears in the January 27 issue of Nature. The plant uses elastic energy stored in the curvature of its leaves, which buckle shut when an insect brushes a hair trigger on the inner leaf surface.

Courtesy of Lakshminarayanan Mahadevan

Above: In a time-exposure photograph, a glass probe (foreground) stimulates a Venus flytrap's leaves to close. Below: Lakshminarayanan Mahadevan demonstrates how the plant springs its trap, even without nerves or muscles.

Photograph by Kris Snibbe / Harvard News Office

The researchers observed, as did Charles Darwin, that the curvature of the plants’ leaves flipped from convex to concave when they closed. They hypothesized that the plant might be exploiting a dynamic instability in the leaf’s structure, flipping from one mode to the other in much the same way one can invert a soft contact lens.

The team painted fluorescent dots on the outer surfaces of leaves and then filmed the plants in action with a high-speed video camera under ultraviolet light, which made the motion of the dots, and hence the leaves, visible. They used mirrors to create two images of the leaf’s dotted surface. “If you have two views of the same object,” Mahadevan explains, “that is sufficient to reconstruct what it looks like in three dimensions.”

He used a combination of theory, modeling, and experiment to show how the plant uses pent-up elastic energy to effect a rapid mechanical closure. Prior research by others had shown that the Venus flytrap moves water inside its leaves to make them open and close, but such changes in cellular turgidity do not take place fast enough to account for the trap’s speedy snap. The secret to the quick closure proved to be mechanical buckling triggered by movements of water in the leaf. The researchers devised equations that describe the action and predict whether a leaf can shut and how fast it will move. Bigger, highly curved leaves snap more quickly.

Mahadevan’s approach is an “old-school, very unfashionable way of doing science that used to be called ‘natural philosophy’ a century ago, where you wondered about everything.” He had been fascinated by the flytrap’s mechanics for some time, but was inspired to actually study the plant “for fun” when a colleague gave him one to grow on his desk. In the freshman seminar he teaches on “The Science of Everyday Life,” he finds connections between the wrinkling of carbon nanotubes and the wrinkling of an elephant’s trunk, and between the formation of mountains and the way fabrics drape and fold. “Just because something is familiar doesn’t mean you understand it,” he observes. “This is the common fallacy that all adults make and no child ever does.”

Editor's update January, 2008Mahadevan's work describing how the leaves of the Venus flytrap can buckle shut fast enough to trap a fly was done together with his student Jan Skotheim (now a postdoctoral fellow at Rockefeller University) and postdocs Yoel Forterre (now an assistant professor of physics at Marseille) and Jacques Dumais (now an assistant professor of biology at Harvard).